For many years the availability of frequencies for radio transmissions was not a problem. However, recently spectral resources have become more and more scarce due to a marked escalation in the public's need for additional channel capacity. The FM modulation techniques in use today require bandwidths which are too wide to permit the allocation of a sufficient number of additional channels in existing bands with present technology. Furthermore, due to a general shortage of suitable spectral resources, the assignment of additional spectrum to these uses appears quite unlikely, despite the significant, demonstrable need. These facts have dictated the need to make more efficient use of the spectrum already available for audio communications. Thus attention has turned to the use of single sideband (SSB) signals for transmissions which can dramatically reduce the spectral space required, particularly for land mobile communications. The use of amplitude-compandored signal sideband (ACSB) signals as an alternative to the more traditional "narrowband" FM thus has been proposed as a possible means of relieving the severe spectrum congestion which hampers effective land mobile communications throughout the world.
The following references describe the potential for single sideband communications (SSB) and propose systems for implementing it and are incorporated herein by reference: